KVM: VMX: Fix host GDT.LIMIT corruption

commit 3444d7da upstream.

vmx does not restore GDT.LIMIT to the host value, instead it sets it to 64KB.
This means host userspace can learn a few bits of host memory.

Fix by reloading GDTR when we load other host state.
Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

arch/x86/kvm/vmx.c

@@ -130,6 +130,7 @@ static u64 construct_eptp(unsigned long root_hpa);
 static DEFINE_PER_CPU(struct vmcs *, vmxarea);
 static DEFINE_PER_CPU(struct vmcs *, current_vmcs);
 static DEFINE_PER_CPU(struct list_head, vcpus_on_cpu);
+static DEFINE_PER_CPU(struct desc_ptr, host_gdt);
 
 static unsigned long *vmx_io_bitmap_a;
 static unsigned long *vmx_io_bitmap_b;
@@ -690,6 +691,7 @@ static void __vmx_load_host_state(struct vcpu_vmx *vmx)
 	save_msrs(vmx->guest_msrs, vmx->save_nmsrs);
 	load_msrs(vmx->host_msrs, vmx->save_nmsrs);
 	reload_host_efer(vmx);
+	load_gdt(&__get_cpu_var(host_gdt));
 }
 
 static void vmx_load_host_state(struct vcpu_vmx *vmx)
@@ -1176,6 +1178,8 @@ static void hardware_enable(void *garbage)
 	asm volatile (ASM_VMX_VMXON_RAX
 		      : : "a"(&phys_addr), "m"(phys_addr)
 		      : "memory", "cc");
+
+	store_gdt(&__get_cpu_var(host_gdt));
 }
 
 static void vmclear_local_vcpus(void)